[Alt builder] Revert clang to version 8

alt-builder/Dockerfile

 FROM amd64/debian:buster
 RUN apt-get update && apt-get install -y wget gnupg && \
-	echo "deb http://apt.llvm.org/buster/ llvm-toolchain-buster main" >> /etc/apt/sources.list && \
-	echo "deb-src http://apt.llvm.org/buster/ llvm-toolchain-buster main" >> /etc/apt/sources.list && \
+	echo "deb http://apt.llvm.org/buster/ llvm-toolchain-buster-8 main" >> /etc/apt/sources.list && \
+	echo "deb-src http://apt.llvm.org/buster/ llvm-toolchain-buster-8 main" >> /etc/apt/sources.list && \
 	wget -O llvm.key "https://apt.llvm.org/llvm-snapshot.gpg.key" && apt-key add llvm.key && \
 	apt-get remove --purge -y wget gnupg && \
 	\
 	apt-get update && \
-	apt-get install -y g++ clang++-9 lld-9 \
+	apt-get install -y g++ clang++-8 lld-8 \
 	                   cmake ninja-build python3 \
-	                   jq clang-tidy-9 cppcheck \
+	                   jq clang-tidy-8 cppcheck \
 	                   libxml2-dev libreadline-dev qtbase5-dev graphviz-dev libjsoncpp-dev libtclap-dev \
 	                   libxml2 libreadline7 libqt5widgets5 graphviz libqt5xml5 libjsoncpp1 && \
 	apt-get autoremove -y && \